Method of diffusion into semiconductor wafers

ABSTRACT

In diffusing an impurity into semiconductor wafers within a silica tube by the use of a heating furnace, a method of diffusion involves the following steps: the semiconductor wafers are inserted into the tube from an inlet thereof, the inlet is sealed by a cap, the interior of the tube is placed under vacuum, and an atmosphere of the impurity is formed. Since, at heating, the tube is closed and no inert gas is fed thereinto, the temperature distribution within the tube is held uniform, and hence the quantities of impurity introduction into the semiconductor wafers are not varied.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of diffusing an impurity intoa semiconductor wafer.

2. Description of the Prior Art

In diffusing an impurity into semiconductor wafers, for example, indiffusing a P-type impurity into silicon wafers by the use of boronnitride (BN), the following is a description of a known method. Wafers(partially oxidized to some extent) of boron nitride (which is a P-typeimpurity source) and wafers of silicon are alternately arranged on aboat. In this arrangement, the boat is inserted into a silica tube.Thereafter, the silica tube is heated from an outer periphery, to form aboron oxide atmosphere inside the silica tube. In this manner, boronoxide is deposited on the silicon wafers, and besides, the impurityoxide is diffused into them. In this case, an inert gas (N₂) is fed inthrough one end of the silica tube, so as to prevent the surfaces of thesilicon wafers from being oxidized.

With this known method, however, the silica tube is in the open stateduring the heating and the inert gas is fed in through one end of thesilica tube, so that the temperature distribution within the silica tubeis not uniform in its lengthwise direction. For this reason, thequantity of impurity introduced into the silicon wafers lined up on theboat is dispersed, i.e., varied, and accordingly, surface specificresistance of the silicon wafers depends on the lined-up position ofeach wafer.

This point will now be described in detail. As stated above, the inertgas is caused to flow in through one end of the silica tube. Even whenthe inflowing inert gas reaches a uniformly heated portion within thetube, it does not yet arrive at the uniform heating temperature.Therefore, the inert gas lowers the temperature of the boron nitride andsilicon wafers at the uniformly heated portion of the tube. In addition,the inert gas disturbs the flow of a boron oxide impurity gas producedfrom the boron nitride. Further, the other end of the silica tube isopen in the foregoing boron deposition and diffusion process.Consequently, the air intrudes into the tube from the open end, oxidizesthe surfaces of the silicon wafers and locally checks the deposition ofimpurity. Due to such various factors, the quantity of impurityintroduction into the silicon wafers varies in dependence on thelined-up position of the wafers on the boat, and a variation in thesurface specific resistance arises.

As a method for compensating for the drawbacks described above, theso-called ampoule diffusion procedure is contemplated in which siliconwafers and an impurity are sealed in a vacuum ampoule, the ampoule isinserted into a silica tube, and the tube is heated to directly vaporizeatoms of the impurity and to diffuse them into the semiconductor wafers.With this method, however, the semiconductor surface becomes rough. Inaddition, it is necessary to especially fabricate the ampoule and tobreak it after completion of the diffusion. Thus, the method isuneconomical and is troublesome in procedure.

SUMMARY OF THE INVENTION

It is therefore the principal object of the present invention to providea method of impurity diffusion into semiconductor wafers whichdiminishes a variation in the surface concentrations of the diffusedlayers in the case of an impurity deposition or diffusion for thesilicon wafers.

Another object of the present invention is to provide a method impurityof diffusion into semiconductor wafers which eliminates the want ofeconomy and the troublesome procedure as in the ampoule diffusion and bywhich the diffusion treatment can be carried out within a silica tubemany times.

In order to accomplish these objects, the present invention comtemplatesa process wherein a diffusion apparatus comprising a diffusion furnacecomposed of a furnace core tube and a heater is used, semiconductorwafers are inserted from an inlet of the furnace core tube, the inlet issealed by a cap, the interior of the furnace core tube is placed undervacuum, an atmosphere of an impurity is formed within the tube, e.g., byheating an impurity source within the core tube or by introducing theatmosphere into the tube, and the impurity is diffused into thesemiconductor wafers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view illustrating an embodiment of theapparatus and the diffusion method of the invention for diffusing animpurity into semiconductor wafer;

FIG. 2 is a diagram illustrating the results of comparative experimentsby the method of the present invention and a prior-art method; and

FIGS. 3 and 4 are elevational views each showing other embodiments ofthe present invention.

PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a diffusion apparatus illustrative of the type in which anembodiment of the method of impurity diffusion into semiconductor wafersis practiced according to the present invention. In the diffusionapparatus shown, a furnace core tube 2 made of silica tube is fitted inan electric furnace 1 including an electric resistance heater. At oneend of the furnace core tube 2, a detachable cap 3 is fitted, e.g., thecap 3 has the structure of a cap nut that is screwed onto the tube 2.This cap is made of a metal such as aluminum or of a ceramic. At a partof the cap 3, there is provided a passage 5 for passing a drawer rod 4therethrough. The drawer rod 4 serves to move a boat for positioningsemiconductor wafers which will be described in greater detail. At theother end of the furance core tube 2, an evacuating port 6 is formedwhich communicates with the interior of the furnace core tube 2 andwhich is connected to a vacuum pump not shown. In this case, the hole orpassage 5 is so constructed that the air may not enter even when theinterior of the furnace core tube 2 is evacuate. For example, rod 4 maybe threaded within passage 5.

With such construction, boron nitride wafers 8 and silicon wafers 9 tobe diffused are arranged on the boat 10 so that the silicon wafers canoppose both faces of the former wafers. The boat 10 is placed at an openend 7 of the furnace core tube 2. Thereafter, the open end 7 is sealedby the cap 3. The interior of the furnace core tube 2 is placed undervacuum through the evacuating port 6 opposite to the open end 7 by theuse of the vacuum pump not shown. With the vacuum state held, the boat10 is moved to the central part 1a of the furnace core tube 2 by thedrawer rod 4, and the deposition and diffusion of impurity are carriedout by the application of heat. After the treatment, the boat 10 ispulled back towards the cap 3 of the furnace core tube 2 by the drawerrod 4, the cap 3 is detached, and the boat 10 is taken out.

With this process, the deposition is effected in the vacuum, so that thenon-uniformity in temperature, as caused by the current of an inert gasdiffering in temperature from the furnace core tube 2, does not arise.Since only the boron oxide (B₂ O₃) impurity gas vaporizing from theboron nitride (BN) is used, the non-uniformity in the atmosphere of theboron impurity gas in the vicinity of the silicon wafers caused by theinflow of the inert gas such as N₂ gas can be avoided. Consequently,unformity is achieved in the concentration of the boron impurity to bediffused into the silicon wafers, and no variation occurs in the surfacespecific resistance

Since the air is perfectly shut out, the oxidation of the silicon wafersis prevented, and the semiconductor wafer surfaces are uniformly coveredwith deposition layers of boron oxide.

After the deposition treatment stated above, the boron oxide formed onthe surfaces of the semiconductor wafers is etched and removed with thediffused impurity left behind. Then, diffusion is carried out in aoxidizing atmosphere again.

FIG. 2 illustrates the results when the method of the present inventionand the prior-art method were experimentally studied. The abscissarepresents the position of each silicon wafer arranged on the boat,while the ordinate represents the surface specific resistances of thesilicon wafer after the deposition and after the rediffusion. Asunderstood from the graph, the prior-art method, indicated at 2a and 2b,exhibits U-shaped characteristics, and the silicon wafers have greatdifferences in the surface specific resistance between the central partof the boat and both end parts thereof. In contrast, according to themethod of the present invention, the surface specific resistances of thesilicon wafers becomes substantially horizontal and rectilinear as shownby curves 2c and 2d, and the resistance is not very different betweenthe central part of the boat and the end parts thereof.

In this embodiment, the boron nitride wafers and the silicon wafers areoppositely arranged on the boat. Since, however, the diffusion length ofmolecules (here, B₂ O₃) is extended in the vacuum, the boron vaporspreads sufficiently within the vacuum system, and it is thereforeunnecessary to oppose the boron nitride wafers and the silicon wafers.Accordingly, boron nitride may be in any desired form and may be locatedat any desired position within the furnace core tube. By way of example,as shown in FIG. 3, a tunnel b of boron nitride is made by a cylinderthereof within the furnace core tube, and the boat 10 on which only thesilicon wafers 9 are erected is inserted therein.

Although, in the foregoing embodiment, boron nitride is used as theimpurity, the boron oxide gas may also be employed. The oxide gas may beintroduced after the interior of the furnace core tube with only thesilicon wafers inserted therein has been evacuated.

Although, in the foregoing embodiment, the boat is inserted from one endof the furnace core tube and is taken out from the same end after thedeposition, it is also possible that, as illustrated in FIG. 4, that asimilar cap 3a is also provided at the other end so as to take out theboat therefrom. It is a matter of course that, in this case, theevacuating port for the vacuum may be provided in the outer periphery ofthe furnace core tube or in either of the caps at both the ends.

Although, in the foregoing embodiments, the interior of the furnace coretube is evacuated by means of the vacuum pump, there will be thepossibility that a slight amount of air will remain to oxidize thesilicon wafers. In this respect, the prevention of the oxidation isincreased in reliability by conducting the evacuating operation afterthe tube has been once filled with an inert gas such as N₂.

With the method of diffusion into semiconductor wafers according to thepresent invention as described above, the variation in the surfaceconcentrations of the impurity diffused layers can be reduced toapproximately 1/5. Since the impurity concentrations at the central partand both the end parts of the boat have substantially no difference,large quantities of semiconductor wafers can be received by increasingthe length of the boat. The number of the semiconductor wafers to betreated at one time of deposition can therefore be increased to be 2 - 3times as large as that treated in the prior art.

It will be understood, that the pressure of the tube may be reduced tonot more than 100 torr and that furnace temperatures on the order offrom 850° to 1150°C. may be employed to effect diffusion of theimpurity. Also, other impurities such as phosphorus and antimony may bedeposited by this method. For example, phosphorus nitride (P₃ N₅) andantimony oxide (Sb₂ O₃) may be used as impurity sources. The conditionsand results of such impurity deposition under vacuum are furtherdescribed in U.S. pat. application Ser. No. 440,358 filed on Feb. 7,1974.

In general, the furnace is heated to the required temperatures for apredetermined time of from 10 to about 120 minutes to promote diffusionof the impurity into the silicon wafers. Also, the silicon wafers areusually from about 30 to 100 mm in diameter and are spaced from about 1to 1.5 mm from each other on the supporting jig, i.e., boat 10. Also, aspreviously mentioned, the pressure in the furnace core tube is reducedto not more than 100 Torr. A preferred reduced pressure range for anembodiment of the type illustrated in FIG. 3 is from 0.1 to 1 Torr.

While the novel embodiments of the invention have been described, itwill be understood that various omissions, modifications and changes inthese embodiments may be made by one skilled in the art withoutdeparting from the spirit and scope of the invention.

What is claimed:
 1. A method for diffusing an impurity into asemiconductor wafer comprising preparing a diffusion furnace composed ofa furnace core tube having an inlet with a releasable closure means anda heater, inserting said semiconductor wafer into said furnace core tubethrough said inlet, closing said inlet by said closure means, creating avacuum in the interior of said furnace core tube, forming an atmosphereof an impurity within said tube, depositing the impurity on a surfaceportion of said semiconductor wafer, and diffusing the impurity intosaid surface portion of said semiconductor wafer in an oxidizingatmosphere.
 2. The method of claim 1, wherein the impurity includesboron, phosphorus and antimony, and said semiconductor wafers are madeof silicon.
 3. The method of claim 1, wherein the atmosphere of animpurity is formed within said tube by initially placing an impuritysource with said tube before making said vacuum and then heating saidtube to an elevated temperature sufficient to vaporize said impuritysource.
 4. The method of claim 1, wherein the vacuum is made in saidtube by attaching a vacuum pump to an opening in said tube and byoperating said pump.
 5. The method of claim 1, wherein the atmosphere ofan impurity is formed within said tube by passing vapors of saidimpurity into said furnace.
 6. The method of claim 1, wherein theatmosphere in said furnace core tube is replaced with an inert gas priorto creation of a vacuum in said furnace core tube.
 7. The method ofclaim 1, wherein the pressure in the furnace core tube during depositionof the impurity on said surface portion is not more than 100 Torr. 8.The method of claim 7, wherein the atmosphere in said furnace core tubeis replaced with an inert gas prior to creation of a vacuum in saidfurnace core tube.
 9. The method of claim 7, wherein the furnacetemperature during deposition of said impurity on said semiconductorwafer is from 850° to 1150°C.
 10. The method of claim 9, wherein theatmosphere in said furnace core tube is replaced with an inert gas priorto creation of a vacuum in said furnace core tube.
 11. The method ofclaim 9, wherein said impurity is boron nitride.